1. Technical Field
The present invention relates to a nitride semiconductor LED (light emitting diode).
2. Description of the Related Art
Nitrides of group-III elements, such as gallium nitride (GaN), aluminum nitride (AlN), etc., exhibit high thermal stability and provide a direct transition type energy band structure, and are hence commonly used as materials in photoelectric elements for blue and ultraviolet light. In particular, blue and green light emitting diodes (LED's) that use gallium nitride (GaN) are utilized in a variety of applications, examples of which include large flat panel color displays, traffic lights, interior lighting, high-density light sources, high-resolution output systems, and optical communication.
The structure of a nitride semiconductor LED may include a substrate, a buffer layer, a p-type semiconductor layer, an active layer, an n-type semiconductor layer, and electrodes. The active layer, where the recombination of electrons and electron holes may occur, can include quantum well layers, expressed by the formula InxGa1-xN (0≦x≦1), and quantum barrier layers. The wavelength of the light emitted from the LED may be determined by the type of material forming the active layer.
The active layer may be of a single quantum well (SQW) structure, which has one quantum well layer, or a multi-quantum well (MQW) structure, which has multiple quantum well layers that are smaller than about 100 Å. The MQW structure in particular can provide higher optical efficiency relative to the electric current and higher light-emission output, compared to the SQW structure.
The optical efficiency of a nitride semiconductor LED may be determined by the recombination probability of electrons and electron holes in the active layer, i.e. the internal quantum efficiency. Various research efforts have focused on improving the internal quantum efficiency by improving the structure of the active layer itself or by increasing the effective mass of the carrier.
Spurred by the increased demands for white LED's, many researchers have worked on developing blue and ultraviolet (UV) LED's based on GaN, and as a result, there have been a geometric growth in the efficiency of blue and UV LED's in the past few years. On the contrary, the lower demands for green LED's have led to a relatively slower growth in green LED development, but it is expected that future developments for multifunctional LED lighting will require efficiency improvements in green LED's as well.
A green LED may generally have a higher indium (In) content compared to a blue or UV LED, and the consequent decrease in crystallinity may severely lower the optical output. While it is necessary to improve the crystallinity of the quantum well layers and quantum barrier layers to resolve this problem, increasing the thickness of the quantum barrier layers, in an attempt to improve the crystallinity, can cause difficulties in the movement of electrons and electron holes required for the emitting of light. Needed therefore is a structure that can improve the crystallinity and at the same time improve the mobility of the electrons and (especially) the electron holes into the light emitting region.
A green LED may have a higher indium content than a blue or UV LED, and this high indium content can cause many defects during the procedure for growing (In)GaN. The defects in the GaN and InGaN layers may lower the light-emitting efficiency of the LED.